Dropdown open close with icon change

HTML:

<ul>
<li class="dropdown">
  <a href="#" style="text-decoration: none" class="dropdown-toggle" data-toggle="dropdown">Dropdown
    <span class="caret"></span>
  </a>
  <ul class="dropdown-menu">
    <li><a href="#">Menu1</a>
    </li>
    <li><a href="#">Menu2</a>
    </li>
    <li><a href="#">Menu3</a>
    </li>
  </ul>
</li>
</ul>

css:
 li.open > a > span {
      border-top: 0px !important;
      border-bottom: 4px dashed !important;
    }

For this code dependencies  are : Bootstrap js, Bootstrap css, jquery